1. Field of the Invention
This invention relates to an optical system for a video camera of the frequency dividing type using a half mirror and a low pass double refraction filter in a portion of the picture taking optical system.
2. Description of the Prior Art
Up to now, in many video cameras, (a half mirror has been used) for automatic focus detection and an optical low pass, double refraction filter has been used in part of their picture taking optical systems. The pickup tube or solid state imaging element (hereinafter called "pickup tube, etc.") in the video camera is arranged so that the plane, containing the incidence-normal of the half mirror for the on-axis ray of light of the picture taking optical system and the optical axis of the picture taking optical system, is either parallel or perpendicular to the horizontal scan line of the pickup tube, etc. For this reason, the finder system, for example, was disposed just above or just beside the video camera. In general, the half mirrors in which the semi-transparent coating may be of metal or dielectric material have different transmission ratios for different components of the incident light on the semi-transparent coating, or P component and S component of the polarized light. On the other hand, a property of the double refraction plate is that an incident ray, when emerging is separated into ordinary and extraordinary rays. The video camera utilizes this property and uses the double refraction plate as an optical low pass filter. Therefore, if the incidence ratios of the P and S components of the light incident upon the double refraction plate differ from each other, the transmission ratios of the ordinary and extraordinary rays of the emerging light also differ from each other. Accordingly, the double refraction plate cannot perform to its utmost ability as an optical low pass filter.
Attempts have been made to eliminate such drawbacks by using a correction plate in front of the double refraction plate as proposed in Japanese Patent No. Sho 54-9499. However, the use of the additional optical element necessitates angularization and also reduces the entire optical system transmittance. Therefore, this method is not desirable.
When the picture taking optical system uses the optical low pass filter in the form of a double refraction plate, mention may also be made of those disclosed in U.S. Pat. No. 3,588,224, Japanese Laid Open Patent No. Sho 51-130116, and Japanese Utility Model Nos. Sho 47-18688, 47-18689 and 47-18690.